1. Field of the Invention
The present invention relates to an external cover pipe adapted to be fitted over a fluid transport pipe made of synthetic resin via an elastic seal. More particularly, the invention relates to an external cover pipe including a plurality of split pipe members divided in the peripheral direction of the pipe which cooperate to compress the elastic seal in association with a fastening operation of fastener means for fastening and joining together the split pipe members disposed adjacent each other in the peripheral direction.
2. Description of the Related Art
The fluid transport pipe with which the external cover pipe of the above-noted type is used is made of synthetic resin, so that this fluid transport pipe provides an advantage of being able to absorb, through its elastic bending deformation, an external force such as a bending moment applied to the fluid transport pipe in a direction transverse to its axis due to an earthquake, differential settlement or the like. As a result, damage to a weak portion in the piping system due to an external force may be limited advantageously.
According to the convention, when the split pipe members of the external cover pipe fitted over the fluid transport pipe via the elastic seal are fastened together by the fastener means, an annular gap is formed between the inner peripheral face of the cover pipe formed of the split pipe members and the outer peripheral face of the fluid transport pipe at each extreme axial end on the outer side of the elastic seal.
For instance, as shown in FIG. 8, in case the external cover pipe 30 is constructed as a branch pipe joint whose one split pipe member 50 integrally forms a branch pipe 7, an opposed pair of elastic seals 20 are fitted and retained to the inner peripheral faces of the split pipe members 50 of the cover pipe 50 for preventing leak of fluid from the gap formed between the outer peripheral face of the fluid transport pipe 1 and the inner peripheral face of the external cover pipe 30 and also from the further gap formed between the adjoined portions of the split pipe members 50. Then, with a fastening operation of the fastener means, these elastic seals 20 are elastically compressed, but still form annular gaps W between the outer peripheral face of the fluid transport pipe 1 and the inner peripheral face of the external cover pipe. More particularly, the presence of the seals 20 forms one elongate annular space communicated with an open hole 8 defined in the peripheral wall of the fluid transport pipe 1 and a pair of short annular spaces opened to the outside at the extreme axial ends on the outer side of the respective seals 20. As a result, elastic bending deformation developed in the peripheral wall portion of the fluid transport pipe 1 located within the branch pipe joint may be absorbed within the extent of the gap W.
In this respect, the amount of elastic deformation of the portion of the fluid transport pipe 1 located within the branch pipe joint 7 is much smaller than that of the entire fluid transport pipe 1. However, because of the formation of the gap W, between the inner peripheral face of the branch pipe joint 7 and the outer peripheral face of the fluid transport pipe 1, for allowing elastic bending deformation of the fluid transport pipe 1, with the elastic bending deformation of the fluid transport pipe 1, the peripheral wall portion of the fluid transport pipe 1 contacting either the peripheral sealing portion of the elastic seal 20 or the inner peripheral face portion of the branch pipe joint 7 adjacent thereto elastically deforms in the radial direction into an oval cross sectional shape, while elastically deforming the elastic seal 20. Then, there inevitably occurs reduction in the tightness (crimping) between the elastic seal 20 and the outer peripheral face of the fluid transport pipe at the portions corresponding to the short-diameter portions of the oval shape, so that the fluid flowing through the open bore of the fluid transport pipe 1 tends to leak through these portions of reduced tightness.